The present invention relates to an anti-reflection member including a light transmittable substrate and an anti-reflection film, and, particularly, to an antireflection member suitable for a cathode ray tube.
Anti-reflection members applied to front panels of display apparatuses or the like have been required to have electrically conductive and light absorptive characteristics for improving the contrasts of display images and the ergonomics of users.
On the other hand, along with the recent tendency to flatten panel glass portions of cathode ray tubes, a thickness of each corner portion of the panel glass portion has come to be increased, with a result that the difference in thickness between the corner portion and a center portion of the panel glass portion has become large. Accordingly, a display apparatus using such a cathode ray tube has an inconvenience that the difference in intensity between the center portion and the corner portion of a screen becomes large.
As electrically conductive and light absorptive anti-reflection members, there have been known those of a type including an anti-reflection film using a transition metal. For example, an anti-reflection film having a multi-layer structure of glass, a film formed from a nitride of a transition metal, and a transparent film has been described in Japanese Patent Laid-open No. Hei 9-156964. Further, an anti-reflection film having a multi-layer structure of glass, a film formed from a nitride of a transition metal, a transparent film, a film formed from a nitride of a transition metal, and a transparent film has been described in Japanese Translations of PCT For Patent No. Hei 6-510328.
These electrically conductive and light absorptive anti-reflection members are intended to realize an antireflection function against light incident from a front surface side of the anti-reflection film. According to these electrically conductive and light absorptive antireflection members, a reflectance of the anti-reflection member against light incident from the front surface side of the anti-reflection film can be reduced to a sufficiently low level from the viewpoint of practical use.
By the way, in a conventional cathode ray tube, a panel glass portion has a transmittance of about 50% at a center portion, and a light absorptive film having an absorption ratio of about 75% is provided on the panel glass portion, so that the total transmittance of the panel glass portion provided with the light absorptive film becomes about 38%, with a result that an image can be displayed on the screen with a good contrast.
In addition, the panel glass portion itself has a light absorptive characteristic.
With respect to a panel glass portion of a cathode ray tube of a type having a curved screen, a corner portion is thicker than a center portion by about 14%, and, accordingly, the transmittance of the corner portion becomes about 92% of that of the center portion.
On the other hand, with respect to a panel glass portion of a cathode ray tube of a type having a flattened screen, a corner portion is much thicker than a center portion by about 33% for the convenience of design of the cathode ray tube. Due to such a difference in thickness between the corner portion and the center portion, the transmittance of the corner portion becomes much smaller than that of the corner portion, more concretely, it becomes about 82% of that of the center portion.
To solve the above problem, for a flattened cathode ray tube, it is required to use a panel glass portion having a high transmittance and to reduce the transmittance of an anti-reflection film. If a flattened cathode ray tube meets these requirements, then the cathode ray tube can improve the transmittance ratio between a center portion and a corner portion of a panel glass portion up to about 95% while keeping the contrast performance of the cathode ray tube. Specifically, for a flattened cathode ray tube, it is required to set the transmittance of a panel glass portion to about 75% and the transmittance of an anti-reflection film to about 50%.
For an anti-reflection film having a multi-layer structure including a light absorptive film, generally, a reflectance on a front surface side of the multi-layer film is quite different from that on a back surface side thereof.
For the electrically conductive and light absorptive anti-reflection members disclosed in the above-described two documents, a reflectance against light incident from a front surface side of the multi-layer film can be made very low; however, a reflectance against light incident from a back surface side of the multi-layer film becomes high. Accordingly, in the case of using such an electrically conductive and light absorptive anti-reflection member as a panel glass portion, light emitted from a phosphor screen side is reflected from an interface between the panel glass portion and the film coated thereon, thereby illuminating the phosphor screen again. As a result, the contrast performance of a display image or the like is significantly degraded, thereby causing the problem that an image is doubly displayed.
To cope with such an inconvenience, an optical filter capable of lowering a reflectance against light incident from a back surface side of a multi-layer film has been provided in xe2x80x9cThin Film Optical Filtersxe2x80x9d, H. A. Macleod, McGRAW-HILL, 2nd Ed. pp. 65-66 (1989). For such an optical filter, however, the reflectance against light emitted from the front surface side of the multi-layer film is only about 10% in a visible light region, and, therefore, it fails to obtain a practical anti-reflection characteristic.
An object of the present invention is to provide an anti-reflection member which has an electrical conductivity and a light absorptivity, and which is capable of not only reducing a reflectance against light incident from a front surface side of an anti-reflection film but also sufficiently reducing a reflectance against light incident from a back surface side of the antireflection film.
To achieve the above object, according to an aspect of the present invention, there is provided an antireflection member including a light transmittable substrate and an anti-reflection film formed on the light transmittable substrate, the anti-reflection film including: a first film formed from a light transmittable material on a principal plane of the light transmittable substrate, the first film having a thickness set in a range of 70 nm or less; a second film formed from a light absorptive material having an electrical conductivity on the first film, the second film having a thickness set in a range of 30 nm or less in accordance with the specific transmittance of the anti-reflection film; a third film formed from a light transmittable material on the second film, the third film having an optical thickness in a range of about an xc2xc-wavelength to an xe2x85x9-wavelength; a fourth film formed from a light absorptive material having an electrical conductivity on the third film, the fourth film having a thickness set in a range of 30 nm or less in accordance with the specific transmittance of the anti-reflection film; a fifth film formed from a light transmittable material having a high refractive index on the fourth film; and a sixth film formed from a light transmittable material on the fifth film, the sixth film having a refractive index smaller than that of each of the third film and the fifth film and also having an optical thickness which is about an xc2xc-wavelength when the wavelength of incident light is about 550 nm.
According to the anti-reflection member having the above configuration, since each of the second film and the fourth film constituting part of the anti-reflection film is made from a light absorptive material having an electrical conductivity, an electrical conductivity is given to the entire anti-reflection film by the second and fourth films.
A transmittance of the anti-reflection film can be changed in a wide range by adjusting the thicknesses of the second film and the fourth film in correlation with each other.
Not only reflection of light incident from a front surface side of the anti-reflection film side, that is, the sixth film side, but also reflection of light incident from a back surface side of the anti-reflection film, that is, the light transmittable substrate, side can be sufficiently prevented.
Further, since the fifth film and the sixth films are formed from materials different from each other in refractive index, the transmittance characteristic of the anti-reflection member can be controlled by suitably adjusting the thicknesses of the fifth and sixth films.
A cathode ray tube having a panel portion provided with such an anti-reflection member can desirably keep the contrast performance of display images.